Our Paladin just hit 5th-level and he's taking the Find Steed spell. The spell is incredibly powerful: a free mount that never disappears unless it drops to 0 hit points (you can resummon it twice per day with your spell slots), and it can attack while the paladin is mounted on it by allowing it to act independently (see: "fight as a seamless unit").
I am willing to let my player have some fun and fulfill his fantasies by taking up a mount that is not listed in the spell description. However, I am terrified the mount that can be summoned will trivialize encounters of the PCs level.
I was thinking of limiting the steed to a particular CR per level of the PC, how exactly can I do this?